Saying it has a 0% chance of happening is a bit careless. You do know you're saying there's no way, no how, not any chance at all that the developers would contemplate having us visit another region again after the Kalos region.
Since there is a precedence for it occurring already (second generation) It's a bit of a careless assumption. They've thought it before, there's always the chance they could think it again. I don't think you have a case to say it has a 0% chance of happening.
It's not like we're talking about them introducing guns to the game.
I don't think we'll revisit an older region...but I also don't think it has a 0% chance of happening. Just a very very small one. Who knows what the developers are thinking sometimes.
If we're going to talk about carelessness, let's consider the logic of what you are suggesting.
1. A train station. Nice concept, and it probably is some kind of train station, maybe even an abandoned train car or a local landmark (I seriously live near such a landmark where I am located). However, the idea that it will be connected through what would be a trip of some 5,000 or more miles? Absolutely no chance. Which brings me to the perceived destination.
2. Heading into Goldenrod. Goldenrod City is part of the Johto region. Johto is connected via Routes 26 and 27 to the Kanto region (in fact those are part of the Kanto region, with the starting home roughly 10 tiles from the border). It would be kind of weird and awkward to have to create blockades to cause one to be unable to get to certain areas (and someone with an Action Replay or somesuch, I guarantee, will wallwalk behind it to basically get stuck), and they will not remake TWO regions into the same game as the new region. There is absolutely NO chance they will do this.
3. Air travel? I'm not ruling out that Unova or another region could be connected by air somehow; however, since Hoenn remakes are due, this is unlikely.
4. Route numbers. Routes 2 and 3 have already been noted on screenshots with nice signs, a feature I really like in fact. If Johto and Kanto are connected, they cannot have Routes 2 and 3 used twice. For this reason, Unova also will not be connected as the Routes would have resumed starting at Route 24, etc.
So my theory is well-founded based on realistic expectations. Please do not come up to me and tell me I'm a careless fool, yes?
Cyclone
